The elasticity of an option where its value increases (for a call) or decreases (for a put) almost one-for-two with the underlying price. As such, it delta is 0.5. For example, a call option with a strike price of 100, while its underlying is currently trading at 130, will have its delta equal to or approaching 0.5. Likewise, a put option with a strike price of 100, while its underlying is currently trading at 70 will have a delta of 0.5 or so. In both cases, the option’s elasticity is said to be at the money.

It is sometimes known by its abbreviated form: ATM elasticity.